Various attempts have been made to provide sensing capabilities in the context of petroleum exploration, production, and monitoring, with varying degrees of success. Recently, these attempts have included the use of fiber optic cables to detect acoustic energy. Because the cables typically comprise optically conducting fiber containing a plurality of backscattering inhomogeneities along the length of the fiber, such systems allow the distributed measurement of optical path length changes along an optical fiber by measuring backscattered light from a laser pulse input into the fiber. Because they allow distributed sensing, such systems are often referred to as “Distributed Acoustic Sensing” or “DAS” systems. One use of DAS systems is in seismic applications, in which seismic sources at known locations transmit acoustic signals into the formation, and/or passive seismic sources emit acoustic energy. The signals are received at seismic sensors after passing through and/or reflecting through the formation. The received signals can be processed to give information about the formation through which they passed. This technology can be used to record a variety of seismic information. Another application range is concerning in-well applications, such as flow- and event detection.
Known DAS assemblies with optical fibers having different acoustic sensitivities are disclosed in UK patent GB 2197953 and U.S. Pat. Nos. 4,297,887 and 4,405,198.
The DAS assembly known from U.S. Pat. No. 4,405,198 comprises twisted optical fibers that may be arranged in parallel with other like fibers and axes twisted at different pitches thereby enabling detection of sound waves over a range of frequencies and their angles of incidence.
While there exists a variety of commercially available DAS systems that have varying sensitivity, dynamic range, spatial resolution, linearity, etc., all of these systems are primarily sensitive to axial strain as the angle between direction of travel of the acoustic signal and the fiber axis approaches 90°, DAS cables become much less sensitive to the signal and may even fail to detect it.
Thus, it is desirable to provide an improved cable that is more sensitive to signals travelling normal to its axis and enables distinguishing this radial strain from the axial strain. Such signals travelling normal to the longitudinal axis of the fiber may sometimes be referred to as “broadside” signals and result in radial strain on the fiber. Sensitivity to broadside waves is particularly important for seismic or microseismic applications, with cables on the surface or downhole.
Furthermore, there is a need to provide an improved method for detecting the direction of acoustic signals relative to a longitudinal axis of fiber optical DAS assembly.